A polymerizable liquid crystal compound having a liquid crystal phase yields a polymer having a function such as optical compensation by polymerization. The reason is that alignment of the liquid crystal compound is immobilized by polymerization. In order to utilize such a function of the polymer, various kinds of polymerizable liquid crystal compounds are developed. However, a sufficient function is not presumably satisfied by a single polymerizable liquid crystal compound. Therefore, an attempt has been made for preparing a composition from several polymerizable liquid crystal compounds to polymerize the composition (see Patent literature Nos. 1 to 4).
With regard to an alignment state of the liquid crystal compound, the liquid crystal compound showing homogeneous alignment, tilted alignment, homeotropic alignment, twisted alignment or the like may be occasionally described simply as “having homogeneous alignment,” “having tilted alignment,” “having homeotropic alignment,” “having twisted alignment” or the like.
In a polymer having homeotropic alignment, a direction of an optical axis is in an nz direction, and a refractive index in the optical axis direction is larger than a refractive index in a direction perpendicular thereto. Therefore, the polymer is classified into a positive C-plate according to an index ellipsoid. When the positive C-plate is combined with any other film having an optical function, the positive C-plate can be applied to improvement of optical compensation of a homogeneously aligned liquid crystal mode such as a so-called in-plane switching (IPS) mode, for example, improvement of viewing angle characteristics of a polarizing plate (see Non-patent literature No. 1, Non-patent literature No. 2, Patent literature No. 5 or Patent literature No. 6).
In the application described above, a polymerizable liquid crystal material may be occasionally laminated onto a support substrate such as a glass substrate or a plastic substrate. Examples of materials used as the plastic substrate include polymers such as triacetylcellulose (TAC), polycarbonate, PET and cycloolefin resins.
In order to allow the polymerizable liquid crystal compound to have homeotropic alignment, when the support substrate is a glass substrate, specific methods include a method for selecting a structure of a polymerizable liquid crystal compound to develop a smectic phase (see Patent literature No. 7), and a method for applying lecithin onto a glass substrate as a homeotropic alignment film (see Patent literature No. 8). Moreover, when a plastic substrate is used as the support substrate, an alignment film is required to be formed onto the support substrate (see Patent literature No. 1 or Patent literature No. 9). So far, the present inventors have found out a polymerizable liquid crystal composition allowing formation of uniform homeotropic alignment on the glass substrate or the plastic substrate even without forming the alignment film onto the support substrate from the polymerizable liquid crystal compound, and having an excellent adhesion (Patent literature No. 10, 11 or 12). However, the compositions are used by mixing with an organic solvent in many cases, and when a support substrate has a significantly weak solvent resistance, a poor coating may be occasionally caused. Therefore, a desire has been expressed for a polymerizable liquid crystal composition allowing a uniform coating without damaging the support substrate having the significantly weak solvent resistance, in which a polymerizable liquid crystal compound uniformly has homeotropic alignment.